Signal transmission through optical fibers has become, or is becoming, the dominant signal transmission mode. The bandwidth characteristics of optical fibers, as well as their relative immunity to certain types of interference and contaminants make them the desirable transmission medium in high capacity trunk lines as well as in lower capacity feeder and distribution lines.
No matter what the intended end use may be, individual optical fibers generally are combined in an optical fiber cable which contains a plurality of such fibers, each of which is protected by at least one layer of coating material. In one configuration, the fibers are assembled into groups which are held together by binder ribbons or tubes to form a cable core. This is generally enclosed in a metallic or plastic tube or jacket which, in the latter case, often contains a strength member. In another configuration, the fibers are arrayed in ribbon form and the core tube contains one or more stacked ribbons.
Regardless of the cable configuration, it is usually necessary that the lengths of fiber cable be spliced at their ends to the ends of other cables, which entails splicing each of the individual fibers in a cable to a corresponding individual fiber in the second cable. To this end, there is provided a splice closure which usually comprises a protective case which contains at least one splice tray which, in turn, has a plurality of splice holders mounted thereon, into which the encased individual fiber splices are inserted and held. The cables are entrant into the case and generally are clamped to each end thereof to reduce the effects of tensile forces on the cables and on the splices. Sufficient amounts of fiber slack must be provided for within the case, such as, for example, half a meter of fiber length so that the individual fibers can be pulled clear of the case to effect the splice. The slack also serves the important function of absorbing tensile forces, thereby isolating the splices from such forces. Because of the delicate and brittle nature of individual glass fibers, they cannot be crimped or bent too sharply, i.e., bent to too small a radius of curvature, which places restraints upon slack storage. Thus, there have been numerous arrangements in the prior art addressing the problem of fiber and slack storage, as exemplified by U.S. Pat. Nos. 5,097,529 of Cobb, et al.; 4,679,896 of Krafcik, et al.; and 4,332,435 of Post.
Inasmuch as, at the splice point, the cable itself is opened up and the base fibers are exposed, the only protection afforded the fibers is provided by the closure, which can provide only one or two layers of protection from the outside environment, the requirements therefor being more stringent than for the cable, which normally provides several layers of protection. The closure must anchor the cables stored therein, and it must be capable of withstanding torsional and axial loads transmitted by the cable to the closure so that the splices are protected from these loads. The closure must also seal the inner and outer sheaths of the cables and maintain the seal integrity under extreme environmental conditions. In addition, the closure must provide adequate fiber storage for slack fiber without damaging the fibers and without increasing signal attenuation. The closure preferably should be capable of storing any type of splice, such as, for example, discrete mechanical, discrete fusion, mass fusion or mass mechanical, or other types while reducing forces that tend to damage the splices. Additionally, the closure should provide adequate grounding and anchoring for the metallic strength members of the cable. The closure should also be capable of accepting high fiber count cables as well as those of low fiber count.
Typically, prior art splice closures are somewhat complex, difficult to assemble, are necessarily bulky, and, in use, difficult to access. As a consequence, they are not economical when used for splicing relatively low count fiber cables, such as, for example, drop cables or CATV applications. Also, when used for low fiber count cables, the bulkiness of the closure makes it difficult to provide adequate storage room, without sacrificing accessibility. This problem of size has heretofore been addressed by simply using a large closure designed primarily for high capacity use, where feasible, or by designing special, smaller closures for low capacity use, which cannot carry or contain large numbers of fibers and splices.
In order to insure protection of the splices from moisture, it is current practice to form the closure out of two mating halves, with a grommet therebetween, and clamp them together. Cable entry is through openings in the grommet, which are usually supplied with inserts which seal the cable and in turn are sealed by the grommet. Such a grommet and insert arrangement is shown, for example, in U.S. Pat. No. 5,472,160 of Burek, et al.. In that arrangement, the grommet, which is of a resilient material suitable for moisture sealing, has, at each end thereof, first and second seal members having bores therein for receiving grommet inserts, which, in turn have bores therein for receiving the cable. The seal members are preferably split longitudinally so that the grommet inserts, with cables extending therethrough, can be inserted in the seal members and be tightly embraced thereby. When the two halves of the housing are clamped together, the cable is tightly embraced, as are the seal members, so that a watertight seal is achieved.
In usage, it has been found that such a sealing arrangement can be vulnerable to a bending or flexing of the cable adjacent the entrance to or exit from the closure, which can, in some instances break the integrity of the seal. There have been various arrangements in the prior art for correcting this effect, one such arrangement being shown in U.S. Pat. No. 5,434,945 of Burek, et al. wherein the closure is encased in a protective shell which, after assembly, is filled with an encapsulant. Such an arrangement insures that the splice closure itself is virtually certain to be moisture proof. However, access to the splices is made more difficult by the presence of the encapsulant, which must be removed to permit such access. For high fiber count cables, limited access, while undesirable, does not necessarily pose too much of a problem. However, for low fiber count cables, where frequent access may often be required, such difficulty of access is undesirable.
In U.S. patent application Ser. No. 08/847,214 of Burek, et al., the disclosure of which is incorporated by reference herein, there is disclosed and claimed a splice closure assembly which incorporates a number of features aimed at overcoming the various short-comings of the prior art arrangements discussed in the foregoing. The basic design of that closure and the components thereof is such that the enclosure is substantially completely moisture proof without requiring an encapsulant or inner and outer closures, affords more than adequate space for storing fiber slack, protects the fiber splices themselves from tensile and other physical forces, is adaptable to different types of splice trays including mass splice trays, and affords a high degree of accessibility to the fiber splices. The splice closure of that application has a floored base portion and a cover portion which define an enclosure having a longitudinal axis and first and second ends. Mounted to the floor are first and second pedestals which hold splice holders or trays in an elevated position above the floor, thereby providing slack fiber storage space below the trays and above the floor.
The Burek el al. enclosure is designed to hold four discrete fiber splice trays or one mass fusion splice tray and one discrete splice tray attached to the top thereof. When only discrete fiber splice trays are used, the maximum fiber count is one hundred and forty-four (144) fiber splices, and when a mass splice tray and one discrete splice tray are used, the maximum count is three hundred and twenty-four (324) fused splices, or one hundred and eighty (180) mechanical splices. However, the present status of the art is such that fiber counts in cables are increasing at rates too fast for development of closures to accommodate them. Thus, four hundred and thirty-two (432) fibers in a cable are now being used on a regular basis and, where the Burek, et al. closure could handle eighty (80%) of all closure applications, that percentage has now decreased. Clearly increased capacity of the Burek, et al. closure is needed.